This is my go to when I’m teaching a new and difficult concept to a horse. It’s very easy for them to get and allows you to mark a very specific action.
I would start by getting him used to the clicker, obviously, and allow him to make that connection that click= good behaviour.
Then, you can start to try to get him to back. Any backwards steps he takes, you click. I might even start this on the ground without the trailer, and then move into the trailer once he’s backing nicely on the ground and understands the clicker. Then, move it to the trailer and treat it like the same concept in a new area.
If he’s anything like my two, they start to have fun with it trying to figure out what they’re supposed to be doing, especially if rewards like treats or pets come after the click.
I won’t say this is a permanent solution but I think it would be a helpful way to ‘back up’ (pun intended) the suggestions already being made.